Pressure to clean up contaminated sites has continued under government regulation which requires removal, reduction, destruction, or stabilization of environmentally hazardous chemical compounds. However, some contaminated soil, sludges and other media are difficult to treat in a cost-effective manner primarily due to the large molecular weight organic chemicals causing the contamination. These chemicals gain a reputation as being "recalcitrant" primarily as a result of fundamental physicochemical properties that make treatment difficult. Examples of solids contaminated with organic chemicals that are difficult to treat cost-effectively include soil contaminated with polyaromatic hydrocarbons (PAH), soil contaminated with manufactured gas plant (MGP) waste, surface soil contaminated with petroleum products such as oils, diesel fuel and bunker oil, and soils contaminated with creosote and wood tars.
Biodegradation (one potential method for remediating such contamination) involves using indigenous or introduced (i.e., non-indigenous) bacteria or other microbes to degrade or digest organic chemicals transported across their cell membranes, thereby producing byproducts such as carbon dioxide gas and water. Although biodegradation works well for small molecular weight compounds, it is very slow for large molecular weight compounds, or multi-ring compounds (e.g., PAHs). It can be difficult to biodegrade the above-noted organic chemicals because the bacteria or other microbes have difficulty transporting large, complex molecules across their cell membranes. Further, these organic chemicals typically exhibit low water solubility, and so are not very mobile. This poor mobility and consequent low concentration that partitions into solution (groundwater or soil pore water) lowers the chances that these large organic molecules are even available to bacteria for efficient biodegradation.
Mueller et al. (U.S. Pat. No. 5,614,410) describes a two stage sequential process for remediating soil or groundwater using a strain of Pseudomonas paucimobilis wherein in the first stage bacteria which degrade low molecular weight polycyclic aromatic hydrocarbons, heterocyclic organic compounds and phenolics are used in a pretreatment step. The remaining high molecular weight compounds are treated in a second step of the process with the P. paucimobilis strain, since the strain is not inhibited by low molecular weight compounds because these are removed in the first pretreatment step.
Besides biodegradation, other processes for remediating organic chemical contamination include volatilization. Volatilization is not an economically realistic option for remediating organic chemicals with large molecular weights because of low vapor pressure and high potential for adsorption to the matrix in which they reside. In other words, volatilization is too slow and expensive. Therefore, soil vapor extraction, which is dependent on high volatility, is incapable of removing these compounds from contaminated soil or waste.
In-situ soil flushing in combination with "pump and treat" is yet another method of treating soil contaminated with these large organics. The technology involves flushing large amounts of site ground water or introduced water through contaminated soil and subsequently recapturing the water with a pumping well. This technology is inefficient for high molecular weight organics because these compounds are only slightly soluble in water.
Another method for remediating recalcitrant organic chemicals in soils and wastes is the "dig and haul" method. This method involves physically digging up the contaminated solids. The contaminated solids must still be properly handled and disposed of once removed from the site. The process is costly.
Still another treatment method is thermal desorption, volatilization and recondensation/collection of vapors, or destruction of gas-phase organics via combustion or UV oxidation. This technology requires extensive materials handling and is equipment intensive. It is typically too costly, especially for small sites.
What is needed is an effective method for treating recalcitrant organic chemicals in contaminated soils and wastes quickly and easily at relatively low cost, wherein the method does not require indigenous or introduced bacteria. It would also be advantageous to have such a method that works on-site without the need to remove soil or waste for treatment elsewhere or to dispose of contaminated soil or waste.